Dev Tool

Regex Tester

Test regular expressions in real time. Matches highlighted instantly, capture groups shown, flags support and a built-in pattern library.

REGULAR EXPRESSION

/ /

g global m multiline i case insensitive s dot all

TEST STRING

REPLACE WITH (optional)

MATCHES

Enter a pattern to see matches highlighted.

Common Patterns Library

^[\w.-]+@[\w.-]+\.[a-z]{2,}$

Email address

https?:\/\/[^\s/$.?#].[^\s]*

URL

^\+?[1-9]\d{7,14}$

Phone number

^\d{4}-\d{2}-\d{2}$

Date (YYYY-MM-DD)

\b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b

IPv4 address

^[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$

UUID

^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$

HEX color

(?=.*[A-Z])(?=.*[0-9])(?=.*[!@#$]).{8,}

Strong password

^[a-zA-Z0-9_-]{3,20}$

Username (3-20 chars)

^\d{5}(-\d{4})?$

US ZIP code

<[^>]+>

HTML tags

\b(\w+)\s+\1\b

Duplicate words

Frequently Asked Questions

What regex flavour does the tester use?

The tester uses JavaScript's built-in RegExp engine, which follows the ECMAScript standard. This is the same engine used in all modern browsers and Node.js. If you are writing regex for Python, PHP, or other languages, minor syntax differences may apply.

How do I test my regex against multiple lines?

Enable the multiline flag (m) in the flags field to make ^ and $ match the start and end of each line rather than the whole string. Use the global flag (g) to find all matches rather than stopping at the first one.

Can the regex tester explain what my pattern matches?

The tester highlights all matches in your test string in real time as you type the pattern. Matched groups are shown separately so you can verify capturing groups are working correctly. For a plain-English explanation of a regex pattern, tools like regex101.com provide detailed breakdowns.

What regex flavour does this use?

JavaScript RegExp (ECMAScript). This is the same engine used in Node.js, browsers, and most frontend frameworks. It supports lookaheads, lookbehinds, named groups and Unicode properties.

What do the flags mean?

g (global) finds all matches not just the first. m (multiline) makes ^ and $ match start/end of each line not just the string. i (case insensitive) treats A and a as the same. s (dot all) makes . match newlines too.

How do I use capture groups?

Wrap part of your pattern in () to create a capture group. Groups are numbered from left to right starting at 1. In the replace field use $1, $2 etc. to insert group content. Named groups use (?<name>...) syntax.

Testing regular expressions safely

A regular expression describes a pattern of text, such as an email address, a date, a log line or a URL. It is easy to get one wrong, because a small change can match far more or far less than you intended. This tester highlights every match as you type, shows capture groups and supports flags, so you can check a pattern against real examples before it goes into code.

Flags worth knowing

g (global) finds every match instead of stopping at the first one.
i (ignore case) treats upper and lower case as the same.
m (multiline) makes ^ and $ match at the start and end of each line, not only of the whole text.
s (dotall) lets . match line breaks as well.

Common mistakes

Missing anchors. \d{4} matches the first four digits of 123456. Use ^\d{4}$ to require exactly four digits.
Greedy matching. <.*> runs from the first < to the last > on the line. The lazy form <.*?> stops at the first one.
Unescaped special characters. A dot matches any character, so example.com also matches examplexcom. Write example\.com.
Runaway backtracking. Nested repeats like (a+)+ can freeze a program on certain input. Keep patterns simple, and never run patterns from untrusted users without a time limit.

Different languages, different rules

Browser-based testers use the JavaScript regex engine. PHP, Python and Java support most of the same syntax but differ in details such as lookbehind and named groups, so test the pattern again in your target language before you ship it.